Over the past 12 years, a continuum of studies has been directed at understanding the hormoneal interrelationships between the fetal, placental and maternal compartments. Most of these studies have utilized primate models, where the similarities to equivalent events in human pregnancies are most extensive. During previous years, we concluded a series of experiments which employed fetal hypophysectomy in rhesus monkeys to establish the importance of fetal hypophyseal secretion for gonadal development in utero; particular attention was given to ovarian and testicular development. During the past year we have demonstrated a synergistic action of fetoplacental estrogens and progresterone in regulating maternal prolactin secretion, and we have developed a fetal cannulation procedure which allows for repeated sampling of fetal blood for determining the relationship between pulsatile LH and FSH secretion and gonadal differentiation. To this we have added studies that test GnRH responsiveness of the term fetus versus neonate at one week.